append button doesn't render with some browser language setting

I recently discovered the problem, that the insert button doesn't work in my firefox.
From what I can see, the append button as the second li-element doesn't get rendered.

After playing around, I discovered that my firefox language setting was in german, which led to the above behavior. After switching to English, the append button gets rendered properly.

Did I miss any configuration to make the whole thing work regardless of the language settings or could this be a bug?